The present invention is directed to a capacitor discharge ignition circuit and method adapted for modification of conventional breaker point ignition systems. The circuit is also adapted to more general application to provide gating of the electronic switch and spark advance responsive to an electrical signal generated by a magneto coil, and to provide force commutation of the electronic switch.
Conventional ignition systems may employ contact breaker points to intermittently make and break a connection between a magneto coil and ground to cause periodic firing of the gap ionization discharge device through a high voltage transformer. Newer ignition systems employing capacitor discharge ignition circuits offer more desirable performance characteristics. However, retrofitting of conventional breaker point systems to convert them to capacitor discharge systems has hertofore been impractical, since such retrofitting may require remachining of the magneto, retiming of the flywheel, provision of triggering coils or removal of the breaker points. Removal of the breaker points prevents reconversion of the system to a breaker point system in the case of an electronic malfunction.
Accordingly, it is an object of the present invention to provide a novel capacitor discharge ignition circuit adapted for modification of conventional breaker point ignition systems, having an electronic switch gated by the electrical signal of a conventional magneto coil.
Another object of the present invention is to provide a novel capacitor discharge ignition circuit which may be incorporated into a conventional breaker point ignition system without removing the breaker points so that the breaker points may be reactivated in case of an electronic malfunction.
In a capacitor discharge ignition system, gating of the electronic switch must be induced at the proper time in the engine cycle. Typically this problem has been addressed by gating the switch responsive to an increase of the magneto coil waveforms to a predetermined voltage or by employing a separate trigger coil located adjacent a magnet carrying flywheel. Gating of the electronic switch responsive to the magneto coil waveform reaching a predetermined voltage may not permit proper spark timing in a retrofitted system without retiming the flywheel. It is accordingly a further object of the present invention is to provide a capacitor discharge ignition circuit, adapted for properly timed gating of the electronic switch responsive to the electrical signal produced by a conventional magneto coil.
In conventional breaker point ignition systems, the system is typically timed to fire a gap discharge ignition device nearly coincident with a peak in the magneto coil waveform. Conventional capacitor discharge circuits are biased to gate an electronic switch responsive to the magneto coil waveform increasing to a predetermined voltage, typically a predetermined negative voltage. However, at higher engine speeds the inductance and capacitance of the circuit may cause the portion of the magneto coil waveform increasing in absolute value, to retreat due to inductive blur. Due to the waveform retreat the predetermined gating voltage may be reached at a relatively later point in time. This waveform retreat may cause a retarding of the spark with higher engine speeds rather than the desired spark advance. Still a further object of the present invention is to provide spark advance in an ignition system comprised of a conventionally timed magneto and a capacitor discharge circuit.
Capacitor discharge ignition systems have achieved force commutation of the electronic switch by means of spark current. However, because of the relatively long time necessary to discharge the capacitor sufficiently to reduce the capacitor voltage below the minimum necessary to maintain the low holding currents typical of conventional electronic switches, commutation may be difficult to achieve in higher speed systems. It is accordingly yet a further object of the present invention to provide a novel method and capacitor discharge ignition circuit which achieve force computation in higher speed systems, where insufficient time is available for the requisite discharge of the capacitor by means of spark current.